1. Field of the Invention:
The invention relates to an apparatus for the determination of the distance traveled by a piston in a cylinder, especially for hydraulic, pneumatic or hydropneumatic equipment such as vibration absorbers, pneumatic springs and hydropneumatic springs, whereby the piston can be moved axially by means of a piston rod in a cylinder filled with at least one damping medium.
2. Description of the Prior Art:
Measurement devices of a general type are known, which comprise capacitors and electronic means as shown in Democratic Republic of Germany Pat. Publication No. DD-PS No. 136 069. These devices are used quite generally for measurement of lengths, whereby in this design, the advantages of a simple capacitive system are combined with the measurement technology advantages of the differential capacitive system. For this purpose, on one hand, a measurement capacitor, whose capacity is changed linearly by the displacement to be measured, is connected to a reference alternating voltage, and on the other hand, a reference capacitor with the same dielectric is connected to an alternating current voltage of the same frequency and opposite phase. The amplitudes of the measurement alternating current voltage may thereby be carried by electronic means so that the alternating current voltage induced in an electrode common to both capacitors becomes zero. Such a measurement apparatus is excessively complicated and is hardly practical for use in hydraulic, pneumatic or hydropneumatic equipment.
Measurement devices are also known for the measurement of a layer thickness such as in British Pat. Specification No. 2 106 651, on which the different layer thicknesses deposited on the piston are measured, so that by means of this process the position of the piston can be determined. Devices of this type cannot be used by themselves in hydraulic, pneumatic or hydropneumatic equipment, such as vehicle suspension systems.
Measurement devices based on ultrasound are also known, where the determination of the distance traveled is made by means of sound propagation and the corresponding computing unit. These systems do not lend themselves to miniaturization, and are sensitive to temperature fluctuations and mechanical interference. In addition, they are technically very complicated and, therefore, very costly.
The known measurement devices based on the induction principle have approximately the same disadvantages as the abovementioned ultrasonic sensors. Measurement devices based on capacitance are mechanically sturdy and can also be manufactured cost-effectively, but they are susceptible to external interference and temperature fluctuations.